My computer cannot read arduino through serial port

Hi, guys.

I have a problem with my Arduino uno board now. Information: 1. My computer cannot see my Arduino board through usb serial port. Neither device manager nor Arduino IDE shows an available port. 2. When I plug the USB cable to my computer, the LED beside the mark "L" on my Arduino board and the LED beside the mark "ON" light up without any blink. The TX/RX LED never lights up.

How can I troubleshoot this problem?

Many thanks,